Many electronic testing applications require a fixture capable of providing multiple simultaneous connections for stimulus signals to a device under test and for connection of response signals to test instruments. Examples include integrated circuit wafer testing, hybrid testing, multichip module testing and printed circuit board testing. In particular, a wafer probe fixture needs interface circuitry close to the probes and perhaps hundreds of signal connections, all in a very confined space. As circuits increase in speed and complexity, the ability to test such circuits at operating speed becomes more difficult. Conventional needle probe wafer fixtures typically cannot be used for frequencies greater than 100 MHz. Probes designed for microwave frequencies (up to 50 GHz) are typically suitable only for low pad count devices. There is a need for probe fixtures providing hundreds of probes with a simultaneous mix of low frequency signals and high frequency signals with bandwidths up to a few Gigahertz.
For high frequencies (&gt;100 MHz), the device under test or the fixture may incorporate transmission lines to minimize signal distortion and attenuation. A common non-coaxial transmission line for short distances within circuit modules is a microstrip. This is a strip conductor and an extended parallel conducting surface separated by a dielectric substrate. Alternative noncoaxial transmission lines include configurations in which a strip conductor is coplanar with extended parallel conducting surfaces (separated by air) and configurations in which a strip conductor is buried within a dielectric substrate having extended parallel conducting surfaces above and below the strip conductor (stripline).
For transmission over longer distances, a coaxial cable may be used. Connection between a coaxial cable and a microstrip is typically made by wire bonding a coaxial connector to pads on a substrate or by soldering a coaxial connector onto the edge of a substrate. Wire bonded or soldered connectors are not easily removable and removal may be destructive. Because of space constraints, edge mounted connectors may limit the number of high frequency connections which can be made. In addition, edge mounted connectors may require undesirable long signal paths. There is a need for temporary connections or easily removable connections for high frequency signals which do not require wire bonding or soldering to a microstrip. In addition, there is a need for high frequency signal connections to the interior of a substrate rather than just the edge. In particular, in integrated circuit wafer testing, there is a need for test fixtures providing numerous temporary connections for low frequency signals and simultaneously providing numerous temporary connections between external coax cables and microstrips which are within the test fixture.
A transition from microstrip to coax creates an unavoidable discontinuity in transmission line impedance which in turn causes signal distortion due to reflections. There is a need for a temporary or easily removable connection having a connection geometry which minimizes reflections. In particular, there is a need for a short path between the microstrip ground plane and the shield on the coax.